Oscillatory power production is a general feature of striated muscle. Enhanced oscillatory power output is correlated with the presence of extensions of the amino terminus of myosin light chains. Homologous extensions exist in myosin essential light chains of vertebrate myocardium and the regulatory light chain of Drosophila jump and flight muscles. The exact function of these protein extensions is unknown, but preliminary results suggest that they augment power during contraction. Our central hypothesis is that the light chain extensions make molecular contacts that help pre-position the motor subunit of myosin near its target zone on actin for optimal interaction and power generation. Light chain constructs will be created in mouse myocardium and flies to assess the extent to which power output is diminished by removing or replacing residues thought to be involved in thin filament interaction. The following hypotheses will be tested: 1) intact ventricular strips lacking the essential light chain N-terminal extension produce lower oscillatory power output at submaximal calcium levels than strips with full length light chains, 2) the essential light chain extension exhibits its effect on power only at in vivo lattice spacing, 3) the light chain extension exerts its effect on power by specific, electrostatic interactions with the thin filament, and 4) comparable alterations of the N-terminal extension of the regulatory light chain in Drosophila flight and jump muscles produce structural and functional phenotypes comparable to those observed in mouse hearts. Interfilament spacing, lattice order, and indices of myosin head alignment will be measured in both intact and demembranated (skinned) preparations using low-angle X-ray diffraction, aided by electron microscopy. Isometric force, unloaded shortening velocity, and dynamic stiffness (oscillatory power output) will be measured in skinned preparations under conditions in which the otherwise swollen lattice is restored by osmotic compression to its in vivo spacing. This research will contribute to understanding and treating human muscle diseases in which power output is compromised.